A conventional milling machine is operable in three axes. A part to be milled is secured to a bed of the milling machine. A milling machine head is movable with respect to the bed in a first direction (e.g., along an X-axis) and is movable with respect to the bed in a second direction orthogonal to the first direction (e.g., along a Y-axis). Usually, the X-axis and the Y-axis are in a horizontal plane parallel to the floor of a factory, machine shop or other facility in which the milling machine is operated. Once the milling machine head is positioned over the portion of the part to be milled, a rotating chuck holding the tool to be used on the part is moved downward to engage the part and to remove selected portions of the part to a depth determined by the positioning of the rotating chuck in the Z-axis direction. The Z-axis direction is orthogonal to both the X-axis and the Y-axis, and, in the conventional setting, the Z-axis is orthogonal to the floor of the facility in which the milling machine is located. In some operations, the movement of the milling machine head or the chuck may occur along one axis at any time. Depending on the type of tool installed in the chuck, the milling machine head may be moved along one or both of the X-axis and the Y-axis at the same time as the position of the chuck is adjusted along the Z-axis in complex milling operations. The operations of multi-axis milling machines are typically controlled by a computer numerical control (CNC) system. In addition to controlling the positioning of the milling machine head and the control of the depth of the chuck, many CNC systems also automatically change the tool to be used from a tool carousel or other tool holder.
Milling machines are also available with 5 axes of movement. Unlike conventional 3-axis milling machines, which provide linear movement in each of three orthogonal directions, a 5-axis milling machine provides two additional movements for the tool chuck. In particular, the tool chuck is rotatable about a first rotational axis (e.g., an A-axis) and is also rotatable about a second rotatable axis (e.g., a B-axis) to enable the positioning of the tool at angles with respect to the part being machined. Typically, the A-axis is parallel to or coincident with the existing Y-axis; and the B-axis is parallel to or coincident with the existing X-axis. Although 5-axis milling machines are commercially available, such machines are more expensive than conventional milling machines. Also, many companies have conventional 3-axis milling machines in place to handle a large portion of the milling projects. Replacing one or more of the 3-axis milling machines with a 5-axis machine is an unnecessary and unacceptable expense for many small companies on a tight capital equipment budget.